Many fuel cells used in the production of electricity contain, sequentially, an electrolyte reservoir plate, an anode chamber, an anode electrode, an electrolyte, a cathode electrode, a cathode chamber, a second electrolyte reservoir plate, and a separator plate. Several of these fuel cells are aligned in electrical series to form a fuel cell stack capable of producing electricity.
During operation of this fuel cell stack, electrolyte migrates both intra-cell and inter-cell due to the migration of ionic species in the electrolyte, thereby decreasing the volume of electrolyte (hereinafter referred to as electrolyte content) in one area of the fuel cell while increasing it in another. In a fuel cell that uses phosphoric acid as the electrolyte, positively charged protons are produced and consumed in each of the cells and the migration of the phosphate ion is effectively equivalent to the migration of the phosphoric acid electrolyte. Not only does the electrolyte migrate from the cathode side electrolyte reservoir plate within one cell of a stack to the anode of that cell, it may also migrate across the separator plate to another cell of the stack. This intra-cell migration can cause electrolyte flooding of the anode, thereby reducing the anode performance and the cell efficiency, while inter-cell migration can cause electrolyte flooding of end fuel cells in a fuel cell stack, thereby reducing performance of the stack and stack efficiency.
As the fuel cell operates, electrical potentials are created across individual phosphoric acid fuel cells and across the stack itself. These potentials are illustrated in FIG. 1 where the electrical potential increases from the anode of cell 1 (A.sub.1) to the electrolyte at the anode of cell 1, decreases through the electrolyte of cell 1 (E.sub.1) between the anode and cathode, and then again increases to the cathode of cell 1 (C.sub.1). The potential then remains virtually constant from cell 1 to cell 2 across the cell 1 separator plate (S.sub.1). Then, again, cell 2's potential increases from the anode (A.sub.2) to the cathode (C.sub.2). This sequence continues through the fuel cell stack to the end cell. Even though, as can be seen at E.sub.1, E.sub.2, and E.sub.3, there is a slight decrease in potential across the electrolyte of each cell, the overall electrical potential of an individual cell increases from the anode to the cathode.
Due to the structure and complexity of a fuel cell stack, both the amount of electrolyte migration from the cathode side electrolyte reservoir plate to the anode side electrolyte reservoir plate of a single cell and the amount of inter-cell migration has been difficult to determine. If the migration over time is monitored in an operating fuel cell, a better understanding of this migration will develop, thereby shedding light upon possible solutions to the migration problem. Consequently, what is needed in the art is a means for monitoring electrolyte content such that the amount of migration of the electrolyte through individual cells and across cells in the fuel cell stack can be determined during fuel cell operation.